This invention relates to pumped waveguide devices with pump power monitors. It is particularly useful as a cladding pumped fiber device.
Pumped waveguide devices are highly useful in optical communication systems. Such devices comprise a pump source, a length of active waveguide and a pump waveguide for transmitting pump light from the source to the active waveguide. The waveguides are typically fibers, and the pump source is usually one or more semiconductor diodes. The active waveguide typically has a rare earth doped core. The devices are used as optical amplifiers and, when provided with an optical resonance cavity, as optical lasers.
In a cladding pumped fiber device the active waveguide is typically a double clad fiber containing a single mode rare earth doped core, and the pump waveguide is a multimode fiber for transmitting pump light to the double clad fiber. The device can be configured into a fiber laser by forming high and low reflectivity Bragg gratings within the double clad fiber to define a laser cavity.
In communications applications, among others, it is important to be able to measure the pump power launched into a cladding pumped device. Such measurement is necessary to monitor the pump source and to correct for power drifts. It is also necessary to accurately test the output.
While there are a variety of approaches to measuring pump power, none are wholly satisfactory. One approach is to place detectors at the back facets of the pump diodes. But the detectors do not measure drifts in the launch of the pump light. For example, the diode output may remain constant but the distribution of light emitted from various modes of the diode may change. Since different modes have different efficiencies of coupling into the active waveguide, the launched power changes without detection.
A second approach involves the use of integrating spheres to detect light scattered from the pump waveguide. But the scattered light is light not guided by the pump waveguide rather than the light launched into the active waveguide. So this approach does not provide a direct measure of the launched light. Moreover, the outermost parts of the mode distribution contribute most strongly to the light scattered out of the pump waveguide but contribute least to the light actually pumping the active waveguide core. Thus measurement of the scattered lightxe2x80x94by integrating sphere or any other meansxe2x80x94varies with changes in mode distribution and does not serve as an accurate measure of launched pump light. Accordingly there is a need for a pumped waveguide device with an improved monitor for measuring pump power.
In accordance with the invention, a pumped waveguide device comprising a pump waveguide and an active waveguide is provided with a pump power monitor by disposing between the pump source and the active waveguide, an indicator waveguide having a core doped with a material sensitive to the pump light. For example the material can fluoresce in response to pump light or absorb pump light and generate heat. Pump power is then accurately measured by the fluorescence or heat from the indicator waveguide. Since the fluorescence or heat is generated in the doped core, the measurement is sensitive to the pump power that will enter the active waveguide and is relatively insensitive to changes in mode distribution. Exemplary embodiments include monitored cladding pumped fiber lasers, amplifiers and light sources.